The HIV-1 capsid protein (CA) plays a critical role in multiple steps of the virus life cycle. It is part of the Gag polyprotein, which is cleaved during maturation to give matrix, CA, and nucleocapsid. So far it is known that CA structure and its effects on core stability are critical for the processes of uncoating, reverse transcription, nucler entry, sites of integration, and assembly, and it is also important for cloaking the DNA product from immune surveillance. HIV-1 CA interacts with host factors including TRIM5a, cleavage and polyadenylation specific factor 6 (CPSF6), nucleoporins 153 and 358 (NUP153, NUP358), MxB and Cyclophilin A. All these diverse functions and interactions are regulated by the structure of a single, remarkably flexible protein, CA, which can thus be termed a biological Swiss Army knife. Although several structures of HIV-1 CA fragments or heavily engineered variants have been solved over the past 20 years, none of these has provided the full atomic details of the critical inter-hexameric contacts that govern capsid stability. The Sarafianos lab recently solved crystal structures of HIV-1 CA, including the elusive structure of the intact native full length hexameric HIV-1 CA. The structures reveal previously unknown molecular details, especially at the 3-fold and 2-fold intra-hexamer interfaces that affect core stability. They demonstrate that the inter-hexamer interfaces are malleable and can change through a newly discovered adaptable hydration layer, which is hypothesized to be critical for core stabilization. They reveal a remarkable plasticity which explains the polymorphism observed in virions. They also validate our experimental system as the most relevant for the study of CA interactions in a native context free of artifacts. Finally, in additional preliminary studies this system has been used to study a) binding of a CA-binding antiviral, b) binding of a host factor peptide, and c) so far three CA mutants that increase or decrease core stability and affect virological functions. Unlike all previous CA structures our structures reveal novel information of the structural changes at the inter-hexamer interfaces and establish a comprehensive framework that will help significantly advance the field through the following Aims: AIM 1: Characterize the structural determinants of increased or decreased core stability AIM 2: Structural interactions of CA with host factors AIM 3: Understanding the mechanisms of CA biological functions using CA-targeting antiviral probes These studies will unravel the structural basis of core stability, provide unique information on how CA structure controls interactions with host factors and small molecule effectors, and may reveal vulnerable structural intermediates of various biological functions.